The invention relates to process for locating and measuring deformations in a civil engineering structure.
During the construction of motorways or railway lines, a great deal of civil engineering work is carried out to support the infrastructure of the road or track. But certain sites comprise unidentified natural or artificial cavities. In this case, unexpected subsidence may occur during the construction of the new road or track, or later during its operation, causing serious accidents due to the overloading of such fragile zones, the infiltration of water, drought and shaking.
To avoid such accidents, it would be necessary to make systematic soundings of the site before carrying out the civil engineering work. Such soundings are indeed carried out in land normally subject to subsidence.
Nevertheless, certain sites, not subject to natural landslide (rock slide) were, in former times, occupied by underground military installations, the existence of which is now unknown.
Systematic soundings are very costly and, in addition, they have to be taken close to each other, under and in the vicinity the proposed road or track to guarantee absolute safety.
Furthermore, even in the absence of cavities in the land, the soil may be subject to uneven settlement under certain sections the road or track following successive wet periods and drought, which may lead to ground subsidence, the amplitude of which is certainly limited, but affects safety particularly when it concerns tracks for high speed trains.
In order to limit the risk of subsidence of infill (embankment), it is common practice to lay sheets (laps) or strips of woven or non woven geosynthetic fabric on the soil and in the infill, to stiffen the infill, in the event of subsidence in the soil. In this case, the threads (yarns) of the geosynthetic fabric are subject to considerable forces which lead to a deformation by elongation of the threads and which may cause them to break, hence the collapse of the infill.
In order to detect deformations in a civil engineering structure, at pre-defined thresholds, FR 2 728 677 has suggested incorporating parallel threads in the geosynthetic fabric, which are capable of transmitting signals and calibrated to pre-determined elongation at break values. The deformation threshold attained is measured by sending signals through the threads and by detecting the presence or absence of response to such signals.
The threads may be electrical wires or optical fibres. The measurement is taken by all or nothing, and location of the deformation at a pre-defined threshold is only possible by placing two sets of parallel threads perpendicular to each other.
The purpose of the invention is to provide a process which provides for the location of the deformation on the one hand and, on the other, the measurement of the elongation of threads where the deformation occurs, before the threads break, using a geosynthetic fabric with a single set of threads, preferably laid parallel to the length of the structure.